The characteristics of speed, torque (turning or twisting force), and power (rate or speed at which work is performed) for a typical internal combustion engine in a motor vehicle such as a motorcycle or a car usually do not match the requirements of the final propulsion component. For example, the range of output of the engine in a motorcycle does not match the range of requirements of the wheels in contact with the road surface. A clutch, disengageably connecting the engine to the transmission, provides the means to apply and remove engine torque to the transmission's input drive shaft.
A typical hydraulic clutch arrangement includes a clutch hand lever placed at the handlebars which actuates a master cylinder. The master cylinder is fluidly coupled to a slave cylinder mounted on or near the engine casing. The slave cylinder in turn actuates a push rod or alternatively a clutch lever which forcibly disengages the clutch. A hydraulic fluid reservoir typically attached at or near the master cylinder and becomes isolated from the system during master cylinder actuation. Spring biasing, integral to the clutch, biases the slave cylinder and master cylinder at rest so that the fluid reservoir may provide relief against environmental changes.
A typical hydraulic clutch is disengaged by depressing the clutch lever which compresses the master cylinder, generating pressure that actuates the slave cylinder, and transmits force along the push rod through to a pressure plate, lifting the pressure plate away from the clutch housing, relieving pressure between the friction and friction bearing elements, resulting in the disengagement of the engine from the transmission. This approach has a number of disadvantages, including the physical effort required to disengage the clutch lever which may lead to rider fatigue. Additionally, careful operation of the clutch lever in conjunction with the gear selector requires a level of concentration that may distract the rider and lead to loss of control. Also, mechanical clearances coupled with non-linear hydraulic effects limit clutch feedback and response, which in turn retards the rider's ability to finely control the clutch.
Many modern vehicles may incorporate a so-called automatic clutch instead of a manually-actuated clutch, such as the one described above, which automatically engages and disengages a friction clutch with some form of actuator.
The automatic clutch suffers from a number of drawbacks. If the automatic clutch fails, the vehicle is inoperable. There is no fail-safe mode of operation that permits the continued operation of the vehicle under those conditions. Additionally, the control system for automatics is not intuitive and may not respond to various driving situations when specific modes of clutch operation are desired. For example, the transmission may shift at a time when the rider of the vehicle does not expect it, which may lead to a loss of control.
In response, the so-called semi-automatic clutch was developed, which included both a manually-actuated clutch in addition to an automatic clutch. The known semi-automatic clutch has a problem when switching between the manual and the automatic modes of operation. During this switching process, when one mode is switched to the other mode during the disconnection of the clutch, the clutch may rapidly be engaged, which may cause unexpected acceleration and a jarring sensation.
In U.S. Pat. No. 6,170,624, a system is proposed to address connection shock. This application discloses a semi-automatic clutch that may prevent connection shock from occurring during the transfer from one mode to another, but only after the connection of the clutch is finished.
A drawback of the current state of the art in semi-automatic clutches is that there is a limitation on when the switch may occur between a manual mode and an automatic mode of operation. This limitation on timing prevents the operator from having the complete freedom to engage the manual override of the clutch at any time during operation of the vehicle.
Another drawback of the current state of the art is the complexity of the current semi-automatic systems. In particular, many alternative systems use a number of isolated hydraulic circuits, which require a separate reservoir for each hydraulic circuit. This complexity may increase the chances of mechanical failure during the prolonged operation that modern vehicles routinely endure, and increase the difficulty and cost of regular maintenance, and repair in the event of a failure.
There is a need to provide a way of switching between operating modes not only when the clutch is engaged, but at any time while the vehicle is operating, smoothly without shock, using a device that contains only one hydraulic circuit and reservoir.